Why Cities Need Forests to Manage Their Water

Cities can thank forests for their water - and for protecting them from floods.

Go to your kitchen sink and open the tap. If you can take a glass, fill it with cool crisp liquid from the faucet, and take a nice big gulp you can likely thank trees for that refreshing drink of water.

Cities often have intricate water systems to ensure - no matter what time of day or year - you have access to that clean glass of water. But at the very beginning of all the pipes and pumps lies a forest!

New York City is a great example of making the most of nature. The city saved money by conserving and restoring the Catskills/Delaware watershed, which supplies 90% of its water. By investing some $1.5 billion to protect the forested watershed, the city avoided building a water filtration plant and saved between $6-8 billion dollars.

A third of the world’s largest cities rely on protected forests for their water. And with a rapidly growing and urbanizing global population, that number will only increase in the future.

So, how do forests manage urban water supply? Here are three ways the trees of a forest ensure quality water continues to flow from your tap, and so much more.

Forests Provide Water.

A forest’s first role in managing a city's water comes from the vapor produced by its trees and vegetation. Water captured, stored, and released by forest canopy is an important factor in regulating precipitation and the evaporation of groundwater.

Unfortunately, if forested watersheds become degraded, the regularity of precipitation is thrown off and rainfall becomes less predictable, leading to fluctuating periods of flooding and drought.

The impact of deforestation on a city’s water supply became abundantly clear in Brazil just four years ago. Riots broke out in Sao Paulo and the surrounding municipalities when a record-breaking drought brought reservoir levels down to just 5% - only a months supply for an area with more than 21 million people.

The drought has been blamed on large-scale deforestation to make room for agricultural production taking place in the nearby Amazon rainforest. Without the trees to regulate precipitation, Brazil’s biggest city nearly found itself without any water at all.

After the rain comes tumbling down, trees stabilize the soil and prevent erosion. The root systems of forest vegetation maintain the forest floor, ensuring sediment and other materials aren’t swept into city water systems.

Without this natural filtration, water flowing out of the taps would be filled with runoff, requiring excessively expensive treatment plants to make the water drinkable.

For example, wildfires in Colorado during the late 1990s and early 2000s destroyed important watersheds, resulting in over $25 million in clean up costs to Denver’s water utility. Denver has since invested in protecting those forested watersheds from wildfires and degradation.

Forests Control Water.

As forests take on all that water they play another vital role for cities: flood control.

Not only are the root systems filtering out pollutants, they are also slowing down the flow of water towards creeks, rivers, lakes, and ultimately cities. Forests require a lot of water to grow, so as rainwater soaks into the soil trees absorb substantial amounts through their roots. This uptake slows water down, stopping it from crashing downstream and flooding cities along the way.

Chopping down large swaths of trees to make way for other land uses makes water absorption impossible and creates a much higher risk of flooding.

Few places have experienced this worse than Jakarta, Indonesia. Fifty percent of Jakarta’s main watershed has been converted to agricultural land or urban development. Without the trees to stop it, throughout the rainy season water fills the many rivers running through the city of more than 10 million people, forcing many to evacuate their homes.

﻿Forests Matter.

Despite being surrounded by pavement, skyscrapers and all necessary amenities, life in the city is drastically improved by the existence of forests, both near and far. They provide potable water, reduce utility costs, and can even save people's lives and livelihoods.